test: include systemd-journald in client test rootfs

test: include systemd-journald in client test rootfs

After commit d36012143c7c ("test: use systemd inside client test rootfs
if available") some tests became racy (for example test 20 on
debian:latest or fedora:latest). There is `made it to the test rootfs!`
in the logs but not the `All OK`.

Following warning can be found in the logs:

```
systemd[1]: systemd-journald.socket: Unit configuration changed while unit was running, and no socket file descriptors are open. Unit not functional until restarted.
systemd[1]: systemd-udevd-kernel.socket: Unit configuration changed while unit was running, and no socket file descriptors are open. Unit not functional until restarted.
systemd[1]: systemd-journald-dev-log.socket: Unit configuration changed while unit was running, and no socket file descriptors are open. Unit not functional until restarted.
```

Include `systemd-journald` in client test root file system when systemd
is used to ensure that systemd-journald continues working.

Hopefully fixes: https://github.com/dracut-ng/dracut-ng/issues/2137